reader: fix json-glib usage

This patch fixes two issues with our json-glib usage.

* Do not atempt to read a string value when the node does not
  hold a string.

* Make sure to call _end_member() when _read_member() fails.

Without these fixes our code will result in segmentation fault.

We were relying on the old buggy behaviour of json-glib that has been
fixed up in:
501c9fb reader: When a read() fails, don't track back on end()

https://bugzilla.gnome.org/show_bug.cgi?id=724927